In recent years, in a mobile communication system such as a cellular phone or a personal handy-phone system (PHS), with an aim to improve a speech quality and reduce line costs, etc., a study has been made to switch a backbone between base stations from a line switching network to an IP network. In a mobile communication system in which the base stations are connected to each other by an IP network, communication paths between the base stations are switched with handover of a mobile station.
FIGS. 14 to 17 are diagrams for illustrating a procedure for a mobile station 74-1 that makes a call connection with a mobile station 74-2 to perform handover from a base station 72-1 to a base station 72-2 in a conventional mobile communication system 70 where a real-time transport protocol (RTP) being one of connectionless transport protocols is applied to a data communication between the base stations. As illustrated in FIGS. 14 to 17, an RTP session 1 (switching source session) is established between the handover source base station 72-1 and the base station 72-3 that makes a radio communication with the mobile station 74-2 before the mobile station 74-1 starts handover, and communication data is transmitted and received by the aid of the RTP session 1 (refer to FIG. 14).
In this example, when the mobile station 74-1 starts handover to the base station 72-2, a new RTP session 2 (switching destination session) is established between the handover destination base station 72-2 and the base station 72-3, and data is transmitted and received by both sessions of the RTP session 1 and the RTP session 2 (refer to FIG. 15). In this state, only data received by the RTP session 1 is used in the mobile stations 74-1 and 74-2. That is, the mobile station 74-1 uses only data received from the handover source base station 72-1, and discards data received from the handover destination base station 72-2. Further, the base station 72-3 transmits only data received by the RTP session 1 to the mobile station 74-2, and discards data received by the RTP session 2.
Upon establishment of the RTP session 2, the base station 72-3 releases the RTP session 1 being the switching source session (refer to FIG. 16). Thereafter, data transmitted or received by the RTP session 2 is used in the mobile stations 74-1 and 74-2 (refer to FIG. 17).
It should be noted that Patent document 1 discloses a technology by which two identical speech sessions are established among two terminals and a handover server located between those terminals at the same time, to thereby realize seamless handover processing. Patent document 1: JP 2007-142786 A